A. The need for band pass charateristics of the Korotkoff sound amplifier. Since the radial (or brachial) pulse has frequency components extending into the audible range, it can be confused with Korotkoff sounds. The sounds from the radial pulse are present from systolic pressure to far below diastolic. Korotkoff sounds are only present when the artery is partially occluded with a pressure between systolic and diastolic pressures. Therefore, for accurate (or even repeatable) measurements of diastolic pressure, stringent requirements exist on the low frequency cutoff of the band pass filter.
Noise from movement occurs in the brachial cuff but is less apparent in our device because of the small area and soft material interface with the skin. Nevertheless, a high frequency cutoff is desired to reduce the noise from movement, muscle tremor, etc. Hence, a band pass filter (not only low pass or high pass) is required to obtain both systolic and diastolic pressure.
B. Korotkoffs' sounds, unlike radial pulses, can be muffled or disappear between systolic or diastolic. There are two reasons that this may occur. One is called the ausculatory gap. Of the five stages of the Korotkoff sounds produced by reducing the pressure from above systolic to below diastolic, the middle stage, in some people is faint and even non-existent due to the characteristics of the blood vessel. The second cause of the muffling Korotkoffs' sounds is venous pooling generally caused in a cuff method by the inflating the cuff too slowly. Because the pressure in the venous system is less than in the arterial system, the cuff occludes the veins first and blood continues to flow in the artery, but not in the veins at cuff pressure between venous and arterial pressure.
The pressure of venous pooling causes the sounds to be altered, sometimes causing them to cease early and sometimes causing them to extend above systolic and below diastolic down to venous pressure (about 5-10 mmHg.). One advantage of our system over the cuff is that it cannot create venous pooling since a whole member is not occluded and therefore, it eliminates this complication.
It is an object of the invention not to be confused by an ausculatory gap. Devices which provide a time window in which the next Korotkoff sounds are to occur are often fooled by the ausculatory gap and therefore provide inaccurate pressure measurement.
C. It is important to establish the reliability of the pressure measurement on each person. It is well known, for instance, that measurement with a cuff on two people with equal blood pressures will be very different if the arm circumference of the two people is different. Instead, to be most accurate, the cuff width must be proportioned to the arm circumference. Similarly, it is the object of this invention to provide an indication of the reliability of the measured pressure. Unlike prior art, this invention does provide an indication of reliability. From the signal picked up by the sound sensing means, two signals arise. (1) Korotkoffs sounds are obtained by filtering as previously described, and (2) sounds arising from the pulse of blood, passing beneath the sensor, are obtained for the purpose of establishing the reliability of the measurements.
Korotkoff sounds are used to determine the systolic pressure and the diastolic pressure. Displaying pressure at each occurrence of Korotkoff sounds indicates to the user the rate at which the user is allowing the pressure to drop. Different from these two purposes, the Korotkoff sounds indicate the pressure of the ausculatory gap, if present. Hence, one of the objects of this invention is to use Korotkoff sounds for the above three purposes in combination. Separate from the above object it is the object of our invention to use sounds obtained from an underlying arterial pulse, these sounds not being Korotkoffs sounds, and these sounds indicating the presence of an underlying artery. It is well known that sounds are produced by an artery and that these sounds exist when an artery is partially occluded, and may extend to pressures far below diastolic. If these sounds are detected when our device is placed in position on the skin then we have an indication that Korotkoff sounds obtained from the same sensor will be reliable. To provide this reliability signal an LED indicator, easily visible to the operator, indicates whether a pulse is being sensed beneath the sensor. Since this pulse can be sensed above systolic and below diastolic, it is different from sensing only Korotkoff sounds. When a pulse is sensed beneath the sensor the operator will see the LED flashing with each pulse and will be able to rely on the obtained pressures. This feature is a significant advantage over the prior arts since prior art has no indication of sensor position with respect to underlying arteries separate from, yet in conjunction with Korotkoff sounds. The greatest advantage is in the measurement of diastolic pressure where it is well known that Korotkoff sounds do not cease abruptly. The amplitude of Korotkoff sounds is related to the distance between the artery and the sensor and therefore, it is related to sensor position if the sensor is not centered over the artery. If the sensor is not centered over the artery, Korotkoff sounds may attenuate too early and the resulting diastolic pressure will be too high. It is an advantage and an object of the device to guard against this error.
D. The geometric shape and materials used in the sensing device are of an advantageous nature. In prior art little, if any, attention has been given to the shape or material of the sensor. Since this device senses both the pulse and Korotkoff sounds, the shape and materials are novel constructions. One object of this invention is a sensor that is optimally suited for application over an artery. The sensor is long enough to occlude a significant portion of an artery, enough to obtain blood pressures, yet not occlude too much making the occlusion force greater than can be applied by hand. The width of the sensor is less than its lenth to allow the sensor to penetrate between tendons, bones, and other obstructing structures. For example, blood pressure can be obtained from the radial artery because this geometry will occlude the artery against underlying bone and penetrate between the radial projection and the contractor tendons of the forearm. This unique geometry allows blood pressure to be taken at many points in the body where an exposed artery is present. Many times an artery is protected by adjacent structures making blood pressure obtained with prior art inaccurate or impossible. The sensor of this invention has a pliable membrane conformable to anatomical structures with a geometric shape of rectangular with rounded corners. This unique combination allows the measuring of blood pressure at locations that are easily accessible and that the prior art has not been able to take advantage of them.